Metabolism of vitamin K and vitamin K 2,3-epoxide via interaction with a common disulfide.

The effects of thiols and sulfhydryl blocking reagents on the reduction of vitamin K to vitamin K hydroquinone and vitamin K 2,3-epoxide to vitamin K and vitamin K hydroquinone catalyzed by rat hepatic microsomes were investigated to determine the mechanism(s) for these reactions. Both vitamin K and vitamin K 2,3-epoxide reductions were catalyzed more effectively with dithiols than with monothiols as the reductant. The sulfhydryl reagent N-ethylmaleimide (NEM) inhibited vitamin K and vitamin K 2,3-epoxide reduction much more effectively when microsomes were initially treated with dithiothreitol (prereduced). In prereduced microsomes iodoacetamide was approximately half as effective an inhibitor of vitamin K and vitamin K 2,3-epoxide reduction as NEM, but in microsomes not prereduced it was more effective. Iodoacetic acid was ineffective as an inhibitor. Vitamin K or vitamin K 2,3-epoxide added to prereduced microsomes blocked subsequent inhibition by NEM of vitamin K and vitamin K 2,3-epoxide metabolism, respectively. Vitamin K added to prereduced microsomes also blocked inhibition by NEM of vitamin K 2,3-epoxide metabolism, and vitamin K 2,3-epoxide addition blocked inhibition by NEM of vitamin K metabolism. Vitamin K did not diminish the rate of vitamin K 2,3-epoxide metabolism, however, nor did vitamin K 2,3-epoxide diminish the rate of vitamin K metabolism. These data establish that exogenous thiol compounds promote the reduction of at least one protein disulfide which participates in the metabolism of vitamin K and vitamin K 2,3-epoxide. Presumably, the resultant sulfhydryl groups are reoxidized to the disulfide form during the metabolism of either vitamin which protects them from reaction with NEM.